Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black can provide rubber compositions with high abrasion resistance. In recent years, as resource saving and energy saving are socially requested, a low heat build-up of tire rubber has come to be requested as well for the purpose of saving fuel consumption of cars. When a low heat build-up is to be achieved by using carbon black alone, it is considered that a compounding amount of carbon black is reduced or carbon black having a large particle diameter is used, but in both cases, it is known that reinforcement, abrasion resistance and grip on a wet road are not prevented from being reduced. On the other hand, it is known to use silica as a filler in order to enhance a low heat build-up (refer to, for example, patent documents 1 to 4). However, particles of silica tend to be aggregated by virtue of a hydrogen bond of a silanol group which is a surface functional group of silica, and a silanol group is inferior in wettability with rubber molecules due to a —OH group having hydrophilicity to deteriorate dispersion of silica into rubber. The kneading time has to be extended in order to improve the above problem. Also, a Mooney viscosity of a rubber composition is increased due to insufficient dispersion of silica into rubber, and a defect that the rubber composition is inferior in processability such as extrusion has been involved therein. Further, since a surface of a silica particle is acidic, silica adsorbs a basic substance used as a vulcanization accelerator in vulcanizing a rubber composition to prevent the rubber composition from being sufficiently vulcanized, so that the defect that the elastic modulus is not enhanced has been involved therein.
A silane coupling agent has been developed in order to improve the above defects, but dispersion of silica has not yet reached a sufficiently high level, and it has been particularly difficult to obtain industrially good dispersion of silica particles. Accordingly, it is tried to mix silica treated a surface thereof with a hydrophobicity-providing agent to accelerate reaction of a silane coupling agent (patent document 5).
Further, it is disclosed in patent document 6 to use hydrophobic precipitated silica. However, since precipitated silica subjected to complete hydrophobicity treatment is used, surface silanol groups to react with a silane coupling agent disappear, and therefore the defect that rubber is not sufficiently reinforced has been involved therein. Further, silica having an increased particle diameter is used in order to improve a low heat build-up, but in that case, the silica is reduced in a specific surface area by increasing a particle diameter to deteriorate a reinforcing property. It is disclosed in patent document 7 to use silica having a specific form, but a low heat build-up and abrasion resistance of a rubber composition are not sufficiently high.
Patent document 1: Japanese Patent Application Laid-Open Hei 6 No. 248116
Patent document 2: Japanese Patent Application Laid-Open Hei 7 No. 70369
Patent document 3: Japanese Patent Application Laid-Open Hei 8 No. 245838
Patent document 4: Japanese Patent Application Laid-Open Hei 3 No. 252431
Patent document 5: Japanese Patent Application Laid-Open Hei 6 No. 248116
Patent document 6: Japanese Patent Application Laid-Open Hei 6 No. 157825
Patent document 7: Japanese Patent Application Laid-Open No. 2006-37046